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That's not periodic. You'll need to extend some boundaries if you're flowing gas over the domain. Set the downstream & "top" boundary as a pressure outlet. 

The issue now is ensuring you have the pressure gradient set correctly up the sides. Fluent uses a "density-operating density" function , so setting the operating density to be exactly the value of the gas phase on the boundary (ie inflow temperature & pressure) reduces the density function to zero: it's explained in the Operating Conditions section of the User's Guide.